According to a known method tire blanks are fabricated by assembling profiled rubber products in a first stage on a first drum of generally cylindrical shape to obtain a sleeve most commonly called a carcass. The carcass usually comprises a carcass reinforcement ply anchored at its two axial ends to circumferential reinforcing rings designed to constitute the bottom zone of the tire, and profiled elements designed to ensure that the tire is airtight or to reinforce particular zones.
In a second stage the carcass is placed on means suitable for transforming the cylindrical tire blank formed by the carcass into a tire blank of toroidal shape.
After having carried out this stage, known as the shaping stage, it is then possible to assemble the elements that constitute the crown zone which comprises, inter alia, the crown reinforcement plies and a tire tread.
This transformation can be carried out by inflating a membrane placed inside the cylindrical sleeve between the two bead reinforcement rings.
More generally, the carcass is arranged on shaping and assembly means comprising two receiving means positioned opposite one another and designed to hold the carcass to be shaped by its beads. These holding means can preserve airtight contact with the said beads. The shaping operation is carried out by increasing the air pressure inside the volume delimited by the holding means and by the tire carcass, and bringing the holding means axially closer together.
The holding means generally comprise a circumferential element that can move radially. In its expanded position this gripping means ensures holding and airtightness at the level of the bead. In its retracted position the said means allow the carcass to be introduced and the tire blank to be extracted.
In addition it is also possible to add means for holding the bottom zone located axially inside the volume delimited by the tire, or even a turn-up membrane arranged axially outside the volume delimited by the tire and designed to hold down the rubber profiled elements previously arranged on an assembly zone of the shaping and assembly drum.
These technologies are described, for example, in the patent EP 468 580 or in the patent DE 39 07 121.
Note that the shaping and assembly means described in those patents are particularly well suited for shaping carcasses whose bottom zone comprises a reinforcement ring around which it is not necessary to turn up the anchoring of the carcass reinforcement ply and the products that constitute the bead while the carcass is being changed from the cylindrical to the toroidal shape.
It can also be advantageous to position the profiled elements designed to form the bottom zone of the tire before introducing the carcass. These products can undergo axial displacements that are uncontrolled and incompatible with the assembly precision sought during the gripping of the carcass.
To that end, the publication EP 492 239 proposes to put in place a circumferential anchoring line capable of preventing any axial slip of the membrane in this zone. This anchoring line is generally located vertically above the zone in which the bead block is arranged. A drum of this type is shown schematically in FIG. 1.
This drum according to the known prior art comprises a central spindle 1 connected to a frame (not shown) and driven in rotation by a motor (not shown) about the axis XX′. The rotation spindle 1 supports two holding means for the beads (2a, 2b) mounted opposite one another relative to the plane of symmetry YY′. Each bead-holding means comprises a support (5a, 5b) which slides axially on the central spindle 1. The sliding supports (5a, 5b) are arranged so as to make airtight contact with the central spindle 1. Each sliding support also comprises an assembly of return segments (9a, 9b) distributed circumferentially around the axis XX′, which can move radially under the action of an axially mobile slider (11a, 11b). The return segments (9a, 9b) comprise a seat for receiving the beads (10a, 10b) and can exert a radial pressure on the carcass beads.
Two circular rings (6a, 7a, 6b, 7b) are fixed to the two axial ends of the sliding support and serve to guide the return segments (9a, 9b) during the radial movement of the said segments.
A turn-up membrane (12a, 12b) is anchored in an airtight manner by its two axial ends (121a, 122a, 121b, 122b) on the radially outer circumference of each of the rings (6a, 7a, 6b, 7b). The rings (7a, 7b) located at the axial end of the sliding support (2a, 2b) support an extender (8a, 8b) on which the lobe of the holding down membrane (12a, 12b) rests.
As shown in FIG. 1, it can be particularly advantageous to anchor the two axial ends of the turn-up membrane (121a, 122a, 121b, 122b) directly on the inner ring (6a, 6b), on the one hand to ensure the airtightness of the part of the bead-holding means that comprises the radially mobile return segments (9a, 9b), and on the other hand to enclose the bead as completely as possible when the turn-up membrane is actuated, so as to assist the pressing against the shaped carcass of the rubber products previously arranged on the lateral extenders (8a, 8b).
The turn-up membrane covers the bead-receiving seat (10a, 10b), which also assists airtight contact between the bead of the tire to be shaped and the receiving zone (10a, 10b) during the radial expansion of the return segments. The membrane can also slide over the bead-receiving seats (10a, 10b).
Each holding means (2a, 2b) is drawn axially along the central spindle 1 by mobile clamps (3a, 3b) which move axially under the action of the rotation of a motorised screw 13 in openings (4a, 4b) formed in the central spindle 1. This axial movement allows control of the mutual approach of the beads during the shaping operation.
Means (not shown) are provided to allow compressed air to be injected into the central space delimited by the central spindle and the bead holding means.
Thus, the shaping drum according to the known prior art comprises bead-holding means (2a, 2b) mounted opposite one another on a central spindle (1), each of the said bead-holding means comprising:                a support (5a, 5b) mounted on and able to move axially relative to the said spindle (1);        an assembly of return segments (9a, 9b) distributed circumferentially around the axis XX′, which circulate radially between two circular rings (6a, 7a, 6b, 7b) fixed on the support (5a, 5b), and comprising a bead-receiving seat (10a, 10b);        a turn-up membrane (12a, 12b) that can slide over the bead-receiving seat, anchored in an airtight manner by its two ends (121a, 122a, 121b, 122b) on the radially outer circumference of the circular rings (6a, 7a, 6b, 7b). The membrane is also anchored mechanically by suitable means (14a, 14b) to the return segments (9a, 9b) along a circumferential anchoring line located between the anchorings of the ends (121a, 122a, 121b, 122b).        
Note, however, that the anchoring line is preferably located opposite the bead heel receiving zone.
This type of drum is particularly suitable for producing tire blanks in which the carcass is not turned around the reinforcement ring during the shaping operation. Axial displacement is blocked in both directions by the circumferential anchoring, and the bottom zone is held in an airtight way during the shaping operation.
On the other hand, when the section of the bottom zone reinforcement ring is circular, the carcass reinforcement ply and the products constituting the bead rotate around the reinforcement ring. It is then appropriate to arrange that the receiving means fitted on the flanges, which ensure airtight contact with the bead, can also be moved in rotation so as to go along with the movement of the bead during the rotation of the bottom zone around the reinforcement ring. In other words, the rotation of the bead around the reinforcement ring of the bottom zone brings about axial sliding of the turn-up membrane.